The present invention relates to a hydraulic brake booster device. Such a device is more particularly intended to be installed in a vehicle, notably a vehicle of the sedan or utility type. It is an object of the invention to address problems with technological evolutions, requirements for space and manufacturing complexity.
In the automotive field, brake booster devices, notably of the pneumatic or electrohydraulic type, are known. The first pneumatic booster devices in practice comprise a pneumatic servo brake, provided with a variable-volume front chamber separated from a rear chamber, which is likewise a variable-volume chamber, by a partition formed by a leaktight and flexible diaphragm and by a rigid skirt plate. The rigid plate drives a pneumatic piston which, via a push rod, bears against a primary piston of a master cylinder of a hydraulic braking circuit, typically a tandem master cylinder. The front chamber positioned on the master cylinder side is connected pneumatically to a source of fluid. The rear chamber, on the opposite side to the front chamber, is placed on the brake pedal side and is connected pneumatically, in a way controlled by a valve, to a source of driving fluid typically air at atmospheric pressure. At rest, that is to say where a driver is not depressing the brake pedal, the front and rear chambers are connected to one another while the rear chamber is isolated from atmospheric pressure. Under braking, the front chamber is first of all isolated from the rear chamber, then air is admitted into the rear chamber. This admission of air has the effect of driving the partition and of pneumatically boosting the braking.
The disadvantage displayed by this type of pneumatic boosting lies in the volumetric ratio of the boost force. Specifically, because the boost force is provided by air at ambient pressure, which is not very high, the booster has to be large enough in size that the boost force will be great. When, for reasons of space, it is not possible to produce chambers with sufficiently large volumes, provision may be made for a number of chambers to be produced in a cascade configuration. Such embodiments are, however, always to the detriment of the space available in the engine compartment of the vehicle.
Further, these devices by way of vacuum source for the front chamber, use a vacuum established in an engine inlet manifold. Now, with modern-day engines, the amount of air admitted is smaller, and the vacuum source becomes less effective. With diesel engine vehicles, acting in this way is not even conceivable.
Also known are electrohydraulic brake booster devices. Typically, an electric motor is connected to a hydraulic pump which injects a hydraulic fluid under pressure into the braking circuits, downstream of the master cylinder, at the time that these circuits are called into action. Control over this electric motor is achieved via measurements of the pressures obtaining in the front and rear chambers of the pneumatic servo brake. Such a solution presents numerous disadvantages.
First, the boost pumps used therefore have to be high-pressure pumps, capable of a constant delivery. In practice, they have to be driven by high-powered motors, typically having a power of 1 kilowatt. Even for a vehicle with a powerful combustion engine, developing 100 kilowatts for example, this brake boosting alone represents 1% of the power developed by the engine, and this is too much.
Technologically, the pumps supply a high pressure. An orifice plate interposed in a return circuit leading to the reservoir, controlled on the basis of the pressure measurements, allows a hydraulic liquid to be injected under variable pressure into the brake circuit. The opening and closing of the orifice plate also present problems of noise and problems associated with the difficulties associated with accurate control.